1. Field of the Invention
The present invention relates to a spectrophotometer which comprises an optical system for spectrally dispersing measurement light, a photodiode array having a plurality of light-receiving elements arranged in corresponding relation to respective spectral light components dispersed by the optical system and each adapted to receive and photoelectrically convert a corresponding one of the spectral light components, a sample chamber disposed in a light path of the measurement light, and a signal readout section for reading out an amount of electric charge accumulated in each of the light-receiving elements of the photodiode array.
2. Description of the Related Art
In a spectrophotometer using a photodiode array having a plurality of light-receiving elements (photodiodes) arranged along a spectral direction, measurement light is spectrally dispersed into its light components with respect to each wavelength region through a wavelength dispersion element, and the spectral light components are entered into the corresponding light-receiving elements. In each of the light-receiving elements, an electric charge is generated and accumulated in an amount corresponding to an amount (i.e., intensity) of the received light component. In a readout operation, the electric charge accumulated in each of the photodiodes is sucked and detected so as to measure an amount of light component incident into each of the light-receiving elements.
A photodiode is saturated when a certain amount of electric charge is accumulated therein, and precluded from accumulating further electric charge. For this reason, an electric-charge accumulation time has heretofore been set in accordance with a specific one of the light-receiving elements which has the highest spectral intensity, to avoid the occurrence of saturation in all the light-receiving elements. In this technique of setting the electric-charge accumulation time in accordance with the specific light-receiving element having the highest spectral intensity, an amount of electric charge to be accumulated in each of the remaining light-receiving elements corresponding to other wavelength regions having relatively low spectral intensities is significantly reduced, and thereby signal strengths in these wavelength regions are lowered.
In the signal readout operation, a readout noise level is constant irrespective of electric-charge accumulation amounts. That is, the lowered signal strength leads to an increase in noise-to-signal ratio which causes larger measurement errors. Thus, in view of reducing the noise-to-signal ratio to suppress measurement errors, it is desirable to allow each of the light-receiving elements to accumulate an electric charge up to an amount closest to its saturation level so as to provide a higher signal strength.
With a view to reducing noises over the entire wavelength range in absorption spectrum measurements, there has been proposed a technique of changing an electric-charge accumulation time for each of a plurality of light-receiving elements in accordance with a background spectrum (i.e., a spectrum of a light source when no sample is set in a sample chamber) by use of a control circuit for independently controlling respective switching cycles of a plurality of readout switches each associated with a corresponding one of the light-receiving elements, so as to allow each of the light-receiving elements to have a different electric-charge accumulation time [see, for example, JP 08-015013A].
This technique is designed to reduce an electric-charge accumulation time for the light-receiving element corresponding to a wavelength region with a relatively high spectral intensity in an emission spectrum of a light source, and increase an electric-charge accumulation time for the light-receiving element corresponding to a wavelength region with a relatively low spectral intensity in the emission spectrum of the light source. Thus, signal strengths in all the wavelength regions can be uniformed. On the other hand, this technique has a problem about structural complication and high cost of the control circuit for independently controlling the switching cycles of the readout switches of the light-receiving elements, due to the need for calculating and setting an electric-charge accumulation time for each of the light-receiving elements, based on the spectrum intensity of the light source and a target spectral intensity.
An ultraviolet (UV)-visible spectrophotometer has a measurement wavelength range of 190 to 1100 nm. In order to cover this wavelength length, the UV-visible spectrophotometer typically employs a light source comprising a deuterium lamp (hereinafter referred to as “D2 lamp”) and a tungsten halogen lamp (hereinafter referred to as “W lamp”) which are designed to be simultaneously turned on. A spectral characteristic of the light source, i.e., a combination of the D2 lamp and the W lamp, is not flat, and, in particular, the D2 lamp has an outstanding bright-line wavelength. Thus, when the D2 lamp and the W lamp are simultaneously turned on, a bright line at 656 nm exhibits a maximum spectral intensity in the visible region, and a light-receiving element corresponding to this wavelength region will be first saturated. While it is contemplated to lower a spectral intensity of the W lamp so as to avoid saturation of the light-receiving element, the use of the W lamp with such a lowered spectral intensity will spoil an intended advantage of simultaneously turning on the D2 lamp and the W lamp.
From this point of view, there has been proposed a technique of lowering an intensity of incident spectrum to a light-receiving element corresponding to a wavelength region exhibiting an outstanding spectral intensity by use of a light intensity-reducing filter provided on the side of a light-receiving surface of a photodiode array [see, for example, JP 05-079451A].
In this technique, for example, when a D2 lamp and a W lamp as a light source are simultaneously turned on, an amount of light to be received by a light-receiving element corresponding to a bright-line wavelength region around 656 nm having a peak in an emission spectrum of the light source is reduced by the light intensity-reducing filter. This makes it possible to extend a time period before saturation occurs in any one of a plurality of light-receiving elements of a photodiode array, so as to increase signal strengths of the remaining light-receiving elements corresponding to wavelength regions other than that subjected to the filter.
In the use of the light source comprising the D2 and W lamps, the highest spectral intensity is exhibited in a wavelength region of 656±4 nm. Thus, a reduction in amount of electric charge to be accumulated in a specific one of a plurality of light-receiving elements which corresponds to this wavelength region allows each of the remaining light-receiving elements corresponding to other wavelength regions to have an increased signal strength and a reduced noise-to-signal ratio. In the technique based on the light intensity-reducing filter, due to restrictions in processing accuracy, positioning accuracy, adjustment tolerance, etc., it is practically obliged to use a wide-range light intensity-reducing filter covering several light-receiving elements corresponding to a wavelength range of about 656±20 nm. Consequently, as shown in FIG. 6, a light intensity is reduced in an excessively wide wavelength range to undesirably lower a spectral intensity.